ago adv.以前。
（辨析）指从现在起往前推算一段时间，通常和一般过去时的动词连用。
（倒证）He lefl a moment ago.他刚走。
There was a pond here three years ago．三年前，这里有一个池塘。
before adv.先前，以前。
（辨析）指过去某时间之前的时间或泛指以前，通常与动同的现在完成时连用，有时也可与动词的过去完成时或一般过去时连用，也可用干具体的一段时间，在间接引语中用before而不用ago.
（例证）Have you heard about it before？你过去听说过这件事吗？
I have never met him before.我以前从未见过他。
He said that he had met the man somewhere before.
他说他以前在什么地方见过这个人。
He was in Italy the week before.他前一个星期在意大利。
formerly adv.以前，从前。
〔辨析〕泛指以前，有时常用于对比。
（例证）Formerly Ted often lingered around a park, with nothing to do.从前，特德时常在公园里闲逛，无所事事。
He was formerly a worker, but now he is a manager of a shop.他曾经是个工人，可现在他是一家商店的经理。
previously adv.以前，先前。
（辨析）与before用法相同，但更加正式。
（例证）I haven't met him previously.我以前没有见过他。
She previously worked as a teacher.她过去当过教师。
Six months previously he had quitted his job．6个月前他辞职了。